generate random sentences from BNF grammars

erratic reads a language syntax definition in the form of a Backus-Naur Form grammar and generates random sentences in that language. A few example use cases:

• generate random computer programs for parser benchmarking;
• generate gibberish text from a simple English grammar;
• generate random BNF grammars for some weird meta-thing.

Get the source code from GitHub or just download the latest version. You can also find it as an npm module.

You can use erratic both in Node.js apps and in modern browsers. Only browsers that implement ECMAScript 5.1 with strict mode are supported. This includes Chrome 13+, Firefox 4+, Safari 5.1+, IE 10+, and Opera 12+ (source).

To use the library in your project:

• in a Node.js app using npm: do npm install erratic and add var erratic = require('erratic') to your scripts.
• in a Node.js app, manually: put erratic.js into your app's node_modules folder and add var erratic = require('erratic') to your scripts.
• in a browser app: put erratic.js somewhere and add the usual <script src="path/to/erratic.js"></script> tag to your HTML. This will create a global object named erratic.

erratic depends on prettybnf, a BNF grammar parser library. If you're not using npm, you'll need to get prettybnf.js yourself and include it in your app:

• in a Node.js app: put prettybnf.js into your app's node_modules folder.
• in a browser app: put <script src="path/to/prettybnf.js"></script> above the <script src="path/to/erratic.js"></script> tag in your HTML.

There are three top-level exports on the erratic object:

{
    version: '0.1.0', // defines your version of the library
    parse: function (grammar) {}, // String -> Object
    generate: function (rules, rule) {}, // (Object, String) -> String
}

The parse function uses prettybnf to parse the provided BNF grammar and build a rules object used for generating sentences. The generate function takes a rules object returned from parse and the name of a rule to generate (ie, a <nonterminal>) and generates a sentence from that rule.

Consider the following grammar, which you might have saved in the file g.bnf:

<list>  ::=  "<" <items> ">"               ;
<items> ::=  <items> " " <item> | <item>   ;
<item>  ::=  "foo" | "bar" | "baz"         ;

To read this grammar from the file, parse it, and generate a random <list>, you might do the following:

// Node.js specific:
var erratic = require('erratic'), fs = require('fs');
var g = fs.readFileSync('g.bnf', 'utf8');

// The grammar is stored in the string g
var rules = erratic.parse(g);
console.log(erratic.generate(rules, 'list'));

This will print out something like <baz <bar <bar <bar>>> foo foo bar foo>.

For more information on the BNF grammar syntax, check out the prettybnf documentation. Additionally, there are some grammar examples in the examples folder of the erratic source repository.

• fork on GitHub
• write code in erratic.js
• add unit tests to test_erratic.js
• make sure all tests and linting pass: npm test
• send me a pull request

Written by Daniel Connelly ([email protected]).

Released under the Simplified (2-clause) BSD License, described here and in the LICENSE file:

Copyright (c) 2012, Daniel Connelly All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.